1. Field of Invention
The invention concerns a device for post-treatment of exhaust gases from internal combustion engines, in particular lean-burn internal combustion engines of motor vehicles.
2. Description of Prior Art
The use of SCR catalysts to reduce the nitrous oxides in an exhaust gas flow from an internal combustion engine is generally known. As part of the selective catalytic reduction (SCR) performed with this SCR catalyst, a substance with directly reducing action is supplied to the exhaust gas flow, such as ammonia or a pre-product which only releases reducing substances in the exhaust gas. The pre-product can for example be a watery urea solution.
In internal combustion engines operated in motor vehicles, nitrous oxide reduction with the SCR process is therefore difficult because firstly fluctuating operating conditions predominate here which makes it difficult to supply the reducing agent in the correct quantities, and secondly for safety reasons the extremely reactive reducing agent ammonia cannot be used directly but must be produced by the decomposition of ammonia precursor substances such as urea, guanidinium formiate, ammonium carbonate, etc.
Also it must be noted that although firstly as high a conversion rate of nitrous oxides as possible is to be achieved, secondly unnecessary emissions of unconsumed reducing agent such as for example ammonia must be avoided.
In connection with the decomposition of urea into ammonia, it is known that under optimum conditions, i.e., at temperatures above 350° C., this takes place in two stages. According to(NH2)2CO→NH3+HNCO  (1)first thermolysis takes place, i.e., the chemical decomposition of urea. Then according toHNCO+H2O→NH3+CO2  (2)hydrolysis occurs, i.e., the catalytic decomposition of iscocyanic add (HNCO) into ammonia (NH3) and carbon dioxide (CO2).
To convert one mole of nitrous monoxide, one mole of ammonia is needed.4NO+4NH3+O2→4N2+6H2O  (3)
The ratio between NH3 and NOx is known as the feed ratio α.α=NH3/NOx  (4)
In an ideal catalyst this means that with a feed ratio of one, all nitrous oxides are reduced so that a 100% NOx conversion is achieved, because for the NOx conversion XNOx:
                              X          NOx                =                                            c                              NOx                ,                0                                      -                          c              NOx                                            c                          NOx              ,              0                                                          (        5        )            where: cNOx,0: untreated NOx emissions [ppm]                cNOx: NOx emissions after catalyst [ppm]        
If the quantity of ammonia supplied exceeds that of the converted nitrous oxides, unconsumed ammonia is emitted. Because of its toxicity, this must be avoided under all circumstances.
To better understand the process at the catalyst, some reaction principles are outlined briefly below.
If we consider the reaction|1|·A1+|2|·A2→|3|·A3+|4|·A4  (6)where: A1, A2: educts                A3, A4: products        v1: stoichiometric factors        v1<0 for educt        v1>0 for productthen this proceeds at a specific speed known as the reaction speed “r” (7). This is defined as the temporal change in the component “i” in relation to the stoichiometric factor. It therefore relates to a reaction equation and is valueless without this being specified.        
                    r        =                              1                          v              i                                ·                                    ⅆ                              n                i                                                    ⅆ              t                                                          (        7        )            where: ni: mole count of component i [mol]                t: time [s]        
For a volume-resistant reaction, the mole count change “dni” can be replaced by the concentration change “dci”:
                    r        =                              1                          v              i                                ·                                    ⅆ                              c                i                                                    ⅆ              t                                                          (        8        )            where: ci; concentration of component i [mol/m3]
If it is not the speed of a particular reaction which is important but the change of a component, then the substance quantity change rate “R” is used.
                              R          i                =                              ⅆ                          c              i                                            ⅆ            t                                              (        9        )            
For the case of N reactions therefore:
                              R          i                =                                            ⅆ                              c                i                                                    ⅆ              t                                =                                    ∑                              j                =                1                            N                        ⁢                                          v                ij                            ·                              r                j                                                                        (        10        )            
To allow better comparison of reaction speed and substance quantity change rate of different catalysts, these are related to representative values such as, e.g., the catalyst mass, the catalyst volume or the phase boundary area.
There are several ways of describing the correlations determining the reaction speed, one of which is the so-called potency method which is used if the reaction mechanism is unknown.r=k·c1m1·c2m2  (11)where k: speed constant of reaction                m: order of magnitude in relation to reactants Ai, miεR        m:        
  m  =            ∑              i        =        1            N        ⁢                  m        i            ⁢              :            total order of reaction
The part orders “mi” of the reactants are normally determined from laboratory measurements.
The speed of a reaction depends not only on the concentration of the educts and their order, but naturally also on the temperature “T”. In the above method this is contained in the speed constant “k”.
                    k        =                              k            O                    ·                      ⅇ                          (                              -                                                      E                    A                                                        R                    ·                    T                                                              )                                                          (        12        )            where kO: frequency or shock factor [mol1-m·s−1]                EA: activation energy [J/mol]        R: general gas constant 8.31 J/molK        
For the substance change speed for NO at SCR catalysts, a so-called formal kinetic method (potency method) can be used in the formRNO=k·cNOm·cNH3n  (13)wherein “m” normally assumes the value “one” and “n” the value “zero”.
In practical terms this means that the substance quantity change rate can be increased by raising the NO concentration, while an increase in NH3 concentration has no effect on this.
If a platinum-containing NO oxidation catalyst is connected before the SCR catalyst to form NO2 2NO+O22NO2  (14)then the SCR reaction can be substantially accelerated and the low temperature activity perceptibly increased.NO+2NH3+NO2→2N2+3H2O  (15)
Since the reducing agent, e.g., on use of the reducing fluid known as AdBlue®, is present in a form dissolved in water, this water must be evaporated before and during the actual thermolysis and hydrolysis, if the temperatures in the two reactions above lie below 350° C. or if heating takes place only slowly, mainly solid, unmeltable cyanuric add is formed by trimerisation of the isocyanic acid, which leads to solid deposits on or even clogging of the SCR catalyst. This can be remedied as described in DE 40 38 054 A1 in that the exhaust gas flow charged with the reducing agent is passed over a hydrolysis catalyst. The exhaust gas temperature at which quantitative hydrolysis is possible can thus be lowered to 160° C. as long as the urea quantities added are not too great. Such an additional hydrolysis catalyst however further increases the cost of the arrangement for exhaust gas post-treatment.
Despite these measures it is often not possible to avoid the formation of cyanuric acid, melamine or other undesirable solid reaction products, in particular if the NH3 precursor substance, such as urea or urea watery solution, and the exhaust gas are not evenly distributed over the entire flow cross section or the quantities added are too great. It is particularly critical here if locally large quantities of reducing agents make contact with the pipe walls or urea decomposition catalysts while at the same time at this point there is a local minimum flow speed. This has the result that the exhaust gas cannot provide sufficiently high heat quantities to ensure a quantitative decomposition of the reducing agent into NH3. Rather at these points, said deposits of undesirable reducing agent decomposition products occur.
This effect is further amplified by the fact that in vehicles only a very restricted construction space is available for preparation of the reducing agent, which means that in particular in the inflow to the catalyst, the inlet lengths are very short, which in turn leads to a very poor equidistribution over the catalyst cross section because of the flow dead zones, cross section changes and/or flow stalling.
All this has the result that the NOx conversion is not usually limited by the actual SCR reaction but by the release of ammonia from its precursor substances.
DE 3604045C1 and EP 362 483 A1 disclose methods of using a periodically fluctuating addition of ammonia instead of a continuous, stationary ammonia addition, in order to raise the NOx conversion rate at the SCR catalyst.
Here briefly more ammonia is added than would be necessary under stationary conditions, in particular the feed ratio here can rise above one, and then the ammonia quantity falls below the quantity necessary under stationary conditions or is even interrupted completely.
The reason for the rise in NOx conversion observed with this process is the inhibition of educts by ammonia, which can be reduced by briefly lowering the NH3 quantity in the exhaust gas and hence on the catalyst surface.
This method cannot however simply be applied to SCR systems which do not use pure ammonia but an ammonia precursor substance, since because of the periodically very great over-supply, usually the decomposition of reaction medium is incomplete and consequently deposits occur in the form of cyanuric acid, melamine, etc.